If you are interested in Vedanta or traditional non-duality, then run (don't walk) and get this beautiful translation of the Upanishads by Eknath Easwaran. Eknath was a professor of English Literature and it shows! The language is lovely, the metaphors create images that are unforgettable.
There are so many poorly written books in the spiritual circles these days. While the content may be fine, the grammatical errors and poor editing can be a tad jarring. Poetry and well-written prose are the language of spirituality to me (well no, silence is--but still..).

The Upanishads are classic texts written thousands of years ago which are perfectly relevant today. In the form of stories, they illustrate the main aspects of non-duality (i.e., the illusion of the false self (doer, ego), the permanence of the true self (awareness, amness, existence), the beauty of maya etc..).

The introduction to the Upanishads by Eknath is simply masterly. He explains non-duality in a nutshell with evidence from recent scientific and pyschological literature, with annotations from Byron and Shakespeare..... his logic is impeccable and vivid images drive home his points. The (much needed) Sanskrit vocabulary of awakening just rolls off the tongue. Sublime.

It occurs to me that there is not a single existential question that I have had (and my hyperactive, analytical mind has had many) that has not been answered by Vedanta. Eknath feels the same way. Vedanta offers knowledge leading to self-realization and the Upanishads form the basis for it. How fortunate we are that they were penned all those years ago providing moksa (liberation) to thousands of people (jivas), then and now.